Fine metallic and ceramic powders (e.g. less than 40 microns in diameter) are in demand for injection molding processes: larger size powders cannot be efficiently used to fabricate parts and components using this manufacturing process. Titanium is one such metallic material that is lately in demand. Examples of ceramic materials includes alumina and zirconia.
The assignee of the instant application owns U.S. Pat. Nos. 3,099,041 and 3,802,816 directed to production of spherical metallic powders using a rotating electrode process. In this "rotary atomization" process, a consumable cylindrical metallic electrode is rotated within a chamber. A non-consumable electrode is spaced from the consumable rotating electrode and an electric arc between the two electrodes melts the end surface of the consumable electrode forming a film which is cast off by centrifugal force forming droplets which cool to form powders that are collected within the chamber. The mean size of Ti-6-4 powder formed by this process, however, is too large for many industrial applications. As a result, the cost of such metallic powders is high due to the need to screen off the larger size powders from the usable powders.
Previous attempts to reduce the size of the powder has focused on increasing the rotational speed of the rotating consumable electrode. A rotational speed limit of between 15,000-18,000 rpm, however, has been reached due to the linear speed limitations of mechanical bearings of such devices and the damage that occurs in the rotary systems due to imbalances of the system during atomization. There has been some attempt to use vaned rotating disks and cryogenic fluids to improve powder yields. See U.S. Pat. Nos. 4,347,199 and 4,419,060. These attempts, however, have not been successfully developed since the disks are often destroyed during the process ruining the atomization equipment and contaminating the powders.
Another method of forming metallic powders is the gas atomization process. Gas atomization is proven capable of producing powders from non-reactive melts such as iron and aluminum with median sizes down to 40 microns or less. Gas atomization, however, has not been successfully applied to producing large quantities of reactive alloys such as titanium because there is no way of containing and delivering the reactive molten metal in a controlled and continuous manner to the gas nozzle. Batch processing using induction melting in a cooled copper hearth, as shown in U.S. Pat. Nos. 5,084,091 and 5,213,610 have been developed for gas atomizing reactive metals, but the resultant powder is expensive due to the small batch sizes of 25 to 50 Kg. Furthermore, the powder sizes are often too large and require extensive screening due to difficulty of controlling the melt delivery to the atomization region.